Fiber optic interferometer arm length difference test system and test method

Abstract

A fiber interferometer arm length difference test system and test method based on random phase noise separation technology, the system includes: a scanning laser, a single-frequency laser, a test optical path, a fiber interferometer to be tested, a signal acquisition module, and a signal processing system. The test optical path is composed of a 2*2 fiber coupler, a fiber circulator, and two wavelength division multiplexers (WDM); the fiber interferometer to be tested is a pair of Michelson interferometers, composed of a 2*2 fiber coupler, a roll of fiber coil and two Faraday reflectors; the signal acquisition module functions mainly include photoelectric conversion, analog signal acquisition and signal analog-to-digital conversion; the signal processing system functions include scanning data acquisition module control and fiber interferometer arm length difference digital signal processing. The present invention can realize high-precision testing of fiber interferometer arm length difference in noise and vibration environments.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber interferometers, and in particular to a test system and a test method for improving the arm length difference test accuracy of an optical fiber interferometer under a vibration or noise environment. Technical Background

[0002] Fiber optic sensors based on fiber interferometer structures can be used to detect underwater acoustic signals, strain, temperature and other information due to their excellent sensitivity and dynamic range. In particular, fiber optic sensors based on Michelson interferometers and Mach-Zehnder interferometers have been widely used in the field of fiber optic sensing due to their easy reuse.

[0003] Since the fiber interferometer uses the optical coherence principle, its maximum arm length difference is limited by the coherence length of the light source. At the same time, the arm length difference of the interferometer directly determines the noise level of the interference signal output by the interferometer. In addition, when the fiber interferometer is multiplexed on a large scale, the arm length difference of the interferometer affects the timing control accuracy of the interferometer output signal. Therefore, in the development process of the fiber interferometer, it is necessary to accurately test its arm length difference to provide data support for the strict control of the interferometer arm length difference to meet the actual application requirements of the fiber interferometer. Compared with white light interferometry, interferometer interference spectrum calculation method and optical time domain reflectometry technology, optical frequency domain reflectometry technology (OFDR) has a test range of hundreds of meters, a spatial resolution of microns, and no test dead zone. It is more suitable for high-precision testing of the arm length difference of general fiber interferometers. OFDR technology has been studied by researchers at home and abroad for many years, and the technology is relatively mature. For example, the OBR4600 instrument developed by LunaInnovation in the United States can achieve a spatial resolution of 40μm within a test range of 70m.

[0004] On the other hand, OFDR technology detects the beat signal between the two transmission signals of the interferometer, obtains the frequency value of the beat signal in the frequency domain, and uses the linear relationship between the frequency of the beat signal and the delay difference between the two arms to obtain the arm length difference of the corresponding fiber interferometer. When there is noise or vibration in the external environment, especially for fiber interferometers with long arm length differences (such as tens of meters) or test arms wrapped around sensitive components (such as elastomers), random phase noise is easily generated along the test arm. When the intensity of the phase noise reaches a certain level, the beat frequency spectrum of the interferometer will be broadened. At this time, the beat frequency and the delay difference no longer form a simple linear relationship, and the accuracy of the measured arm length difference cannot be guaranteed by conventional calculations. There is currently little research on the random noise problem caused by external environmental noise or vibration in OFDR testing technology, and no relevant results have been shown.

[0005] Therefore, in order to achieve high-precision test of the arm length difference of the fiber optic interferometer in a noisy or vibration environment, it is necessary to design a new arm length difference test system and test method. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a fiber interferometer arm length difference testing system and demodulation method. Based on OFDR technology, dual laser detection technology and random phase noise separation method are adopted to achieve high-precision test of the fiber interferometer arm length difference in a vibration or noise environment.

[0007] The technical solution of the present invention is as follows:

[0008] A fiber interferometer arm length difference test system, characterized in that it includes: a scanning laser, a single-frequency laser, a test optical path, a data acquisition module and a signal processing system; the test optical path includes a 2*2 fiber coupler, a fiber circulator, a first wavelength division multiplexer and a second wavelength division multiplexer;

[0009] The linear wavelength scanning light output by the scanning laser and the single wavelength light output by the single frequency laser are combined by the 2*2 optical fiber coupler, input through the first port of the optical fiber circulator, and then input into the optical fiber interferometer to be tested through the second port. After being reflected by the optical fiber interferometer to be tested, one path returns along the original path, inputs through the second port of the optical fiber circulator, and then outputs to the first wavelength division multiplexer through the third port of the optical fiber circulator, and the other path directly inputs the second wavelength division multiplexer; the interference signals output by the first wavelength division multiplexer and the second wavelength division multiplexer are converted into digital signals by the data acquisition module, and then demodulated by the signal processing system.

[0010] The scanning laser is a narrow line width wavelength linear scanning laser.

[0011] The single-frequency laser is a narrow-linewidth single-frequency laser, and its wavelength is not within the wavelength scanning working range of the scanning laser.

[0012] The operating wavelengths of the first wavelength division multiplexer and the second wavelength division multiplexer are both the same as the wavelength of the single-frequency laser.

[0013] The optical fiber interferometer to be tested is a Michelson optical fiber interferometer or a Mach-Zehnder optical fiber interferometer.

[0014] The data acquisition module comprises a photoelectric conversion module, an analog signal acquisition module and a signal analog-to-digital conversion module in sequence.

[0015] The signal processing system includes acquisition module control and fiber interferometer interference signal digital processing.

[0016] A method for testing the arm length difference of a fiber interferometer using the above-mentioned fiber interferometer arm length difference testing system comprises the following steps:

[0017] Step 1: An interference signal outputted by the fiber interferometer to be tested is divided into a first interference signal I1 and a second interference signal I2 after passing through a fiber circulator and a first wavelength division multiplexer in sequence. The first interference signal I1 is generated by the output light of the scanning laser passing through the fiber interferometer to be tested, and is expressed as The second interference signal I2 is generated by the output light of the single-frequency laser through the fiber interferometer to be measured, and is expressed as Where A1 and A2 are the amplitudes of the interference signal, n is the refractive index of the optical fiber, L is the arm length difference of the optical fiber interferometer, c is the speed of light, is the phase noise, α is the phase difference between the two output ends of the interferometer;

[0018] Another interference signal output by the optical fiber interferometer to be tested is divided into a third interference signal I3 and a fourth interference signal I4 after passing through the second wavelength division multiplexer. The third interference signal I3 is generated by the output light of the scanning laser through the optical fiber interferometer to be tested, and is expressed as The fourth interference signal I4 is generated by the single-frequency laser output light through the fiber interferometer to be measured, and is expressed as Where B1 and B2 are the amplitudes of the interference signals;

[0019] Step 2: The four interference signals pass through the data acquisition module to form digital signals and then enter the signal processing system;

[0020] Step 3: The signal processing system performs signal processing on the four interference signals, as follows:

[0021] ① Normalize the 4-channel interference signals to obtain normalized interference signals I1′~I4′, and their mathematical expressions are:

[0022]

[0023] ② Calculate the difference F, the formula is as follows:

[0024]

[0025] ③ Perform FFT conversion on F to obtain the frequency value corresponding to the maximum value of the signal spectrum amplitude The fiber interferometer arm length difference L can be obtained.

[0026] Compared with the prior art, the beneficial effects of the present invention are: based on OFDR technology, dual laser detection technology and random phase noise separation algorithm are adopted to eliminate the influence of optical fiber random phase noise on the spectrum broadening of the interference signal, and improve the test accuracy of the arm length difference of the optical fiber interferometer in a vibration or noise environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the overall framework diagram of the fiber optic interferometer arm length difference testing system of the present invention.

[0028] Figure 2 Schematic diagram of the test optical path structure in an example of the present invention.

[0029] Figure 3 This is a flow chart of the digital signal processing of the fiber interferometer arm length difference in an example of the present invention.

[0030] Figure 4 These are the interference signals I1 and I3 carrying random phase noise corresponding to the scanning laser in the example of the present invention.

[0031] Figure 5 These are the interference signals I2 and I4 carrying random phase noise corresponding to the single-frequency laser in the example of the present invention.

[0032] Figure 6 This is a spectrum diagram of the reflected beat frequency carrying random phase noise in an example of the present invention.

[0033] Figure 7 This is a spectrum diagram of the reflected beat frequency after the random phase noise is eliminated in the example of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. However, the following embodiments are only used to explain the present invention, and the protection scope of the present invention is not limited to the following embodiments.

[0035] A fiber interferometer arm length difference testing system of the present invention, such as Figure 1 , including a scanning laser 1, a single-frequency laser 2, a test optical path 3, a fiber interferometer to be tested 4, a data acquisition module 5, and a signal processing system 6.

[0036] The scanning laser is Figure 1 Shown is a narrow linewidth wavelength wide range scanning laser with a coherence length greater than 100m and a wavelength scanning range greater than 5nm.

[0037] The single frequency laser Figure 1 Shown is a narrow linewidth single-frequency laser with a coherence length greater than 100m and an operating wavelength of 1548nm.

[0038] The test optical path is as follows Figure 2 As shown, it includes a 2*2 optical fiber coupler 7, an optical fiber circulator 5, and two WDMs 9 and 10.

[0039] The operating wavelength of the WMD is about 1548 nm.

[0040] The optical fiber interferometer to be tested is a pair of Michelson interferometers, the arm length difference is about 9m, and the phase difference between the two output ends of the interferometer is about 90°.

[0041] The functions of the data acquisition module include photoelectric conversion, analog signal acquisition, 12-bit signal analog-to-digital conversion, and a sampling rate greater than 20 MHz.

[0042] The signal processing system performs digital signal processing on the digital signal input from the data acquisition module.

[0043] The signal processing system includes acquisition module control, fiber optic interferometer interference signal digital processing, etc.

[0044] The scanning laser control includes setting the laser wavelength range to 1536-1540 nm, setting the laser scanning rate to 40 nm / s, and setting the laser output power to 8 mW.

[0045] The data acquisition module control includes setting the sampling rate to 20 MHz / s, setting the sampling trigger type to rising edge trigger, and setting the sampling data volume to 1M.

[0046] The fiber optic interferometer arm length difference test system performs a method for testing the fiber optic interferometer arm length difference, characterized in that the method comprises the following steps:

[0047] Step 1: An interference signal outputted by the fiber interferometer to be tested is divided into a first interference signal I1 and a second interference signal I2 after passing through a fiber circulator and a first wavelength division multiplexer in sequence. The first interference signal I1 is generated by the output light of the scanning laser passing through the fiber interferometer to be tested, and is expressed as The second interference signal I2 is generated by the output light of the single-frequency laser through the fiber interferometer to be measured, and is expressed as Where n = 1.4681, L ≈ 9m, α = 90°;

[0048] Another interference signal output by the optical fiber interferometer to be tested is divided into a third interference signal I3 and a fourth interference signal I4 after passing through the second wavelength division multiplexer. The third interference signal I3 is generated by the output light of the scanning laser through the optical fiber interferometer to be tested, and is expressed as The fourth interference signal I4 is generated by the single-frequency laser output light through the fiber interferometer to be measured, and is expressed as

[0049] Step 2: The four interference signals pass through the data acquisition module (5) to form digital signals and then enter the signal processing system;

[0050] Step 3: The signal processing system performs signal processing on the four interference signals, as follows:

[0051] ① Normalize the 4-channel interference signals to obtain normalized interference signals I1′~I4′, and their mathematical expressions are:

[0052] ② Calculate the difference F, the formula is as follows:

[0053]

[0054] ③ Perform FFT conversion on F to obtain the frequency value corresponding to the maximum value of the signal spectrum amplitude The fiber interferometer arm length difference L can be obtained.

Claims

1. A fiber interferometer arm length difference test system, characterized in that: include: A scanning laser (1), a single-frequency laser (2), a test optical path (3), a data acquisition module (5) and a signal processing system (6); the test optical path (3) comprises a 2*2 optical fiber coupler (7), an optical fiber circulator (8), a first wavelength division multiplexer (9) and a second wavelength division multiplexer (10); The linear wavelength scanning light output by the scanning laser (1) and the single wavelength light output by the single frequency laser (2) are combined by the 2*2 optical fiber coupler (7), input through the first port of the optical fiber circulator (8), and then input through the second port to the optical fiber interferometer (4) to be tested. After being reflected by the optical fiber interferometer (4) to be tested, one path returns along the original path, inputs through the second port of the optical fiber circulator (8), and then outputs to the first wavelength division multiplexer (9) through the third port of the optical fiber circulator (8), and the other path directly inputs the second wavelength division multiplexer (10); the interference signals respectively output by the first wavelength division multiplexer (9) and the second wavelength division multiplexer (10) are converted into digital signals by the data acquisition module (5), and then demodulated by the signal processing system (6); The operating wavelengths of the first wavelength division multiplexer (9) and the second wavelength division multiplexer (10) are both the same as the wavelength of the single-frequency laser (2).

2. The fiber interferometer arm length difference test system according to claim 1, characterized in that: The scanning laser is a narrow line width wavelength linear scanning laser.

3. The fiber interferometer arm length difference test system according to claim 1, characterized in that: The single-frequency laser is a narrow-linewidth single-frequency laser, and its wavelength is not within the wavelength scanning working range of the scanning laser.

4. The fiber interferometer arm length difference test system according to claim 1, characterized in that: The optical fiber interferometer to be tested is a Michelson optical fiber interferometer or a Mach-Zehnder optical fiber interferometer.

5. The fiber interferometer arm length difference test system according to claim 1, characterized in that: The data acquisition module comprises a photoelectric conversion module, an analog signal acquisition module and a signal analog-to-digital conversion module in sequence.

6. The fiber interferometer arm length difference test system according to claim 1, characterized in that: The signal processing system includes acquisition module control and fiber optic interferometer interference signal digital processing.

7. A method for testing the arm length difference of a fiber interferometer using the fiber interferometer arm length difference testing system according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: Step 1: An interference signal outputted by the optical fiber interferometer (4) to be tested is divided into a first interference signal I1 and a second interference signal I2 after passing through an optical fiber circulator (8) and a first wavelength division multiplexer (9) in sequence. The first interference signal I1 is generated by the output light of the scanning laser (1) passing through the optical fiber interferometer (4) to be tested, and is expressed as The second interference signal I2 is generated by the output light of the single-frequency laser (2) through the optical fiber interferometer (4) to be measured, and is expressed as Where A1 and A2 are the amplitudes of the interference signal, n is the refractive index of the optical fiber, L is the arm length difference of the optical fiber interferometer, c is the speed of light, is the phase noise, α is the phase difference between the two output ends of the interferometer; Another interference signal outputted by the optical fiber interferometer (4) to be tested is divided into a third interference signal I3 and a fourth interference signal I4 after passing through a second wavelength division multiplexer (10). The third interference signal I3 is generated by the output light of the scanning laser (1) passing through the optical fiber interferometer (4) to be tested, and is expressed as The fourth interference signal I4 is generated by the output light of the single-frequency laser (2) through the optical fiber interferometer (4) to be measured, and is expressed as Where B1 and B2 are the amplitudes of the interference signals; Step 2: The four interference signals are transformed into digital signals after passing through the data acquisition module (5) and input into the signal processing system (6); Step 3: The signal processing system (6) performs signal processing on the four interference signals, as follows: ① Normalize the 4-channel interference signals to obtain normalized interference signals I'1~I'4, whose mathematical expressions are: ② Calculate the difference F, the formula is as follows: ③ Perform FFT conversion on the difference F to obtain the frequency value corresponding to the maximum value of the signal spectrum amplitude The fiber interferometer arm length difference L can be obtained.

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